Potassium deficiency

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Characteristics and occurrence

 

Root crops have a high requirement for potassium (K) compared with cereals, as the content of potassium in the harvested roots is high. A 20 t/ha sweetpotato crop removes approximately 100 kg K/ha in the storage roots, and considerably more is removed if both roots and vines are harvested. Even soils which are naturally high in potassium may become depleted after several successive crops.

In contrast to nitrogen or phosphorus deficiencies, potassium deficiency tends to have a much greater effect on storage root yield than on the growth of the tops. Therefore, correction of an apparently mild deficiency may result in large yield increases.

Symptoms

As with other major nutrients, potassium deficiency can cause substantial growth reduction before specific symptoms develop. In the field, symptoms often develop after two to three months, when the expanding storage roots begin to place increased demand on potassium supplies. Yellowing appears on the oldest leaves, while the youngest leaves retain a normal colour, size and texture. If young leaves are normally purple, this pigmentation may be reduced in K deficient plants.

On the oldest leaves, yellowing occurs in marginal and interveinal zones. Brown necrotic lesions develop within the yellow parts and eventually spread to cover the entire leaf blade. Cultivars vary in the pattern in which lesions spread. In some, spreading is predominantly from the margins to interveinal zones, in others lesions are initiated in interveinal regions nearer the midrib, and some may spread with relatively little regard for veinal distribution. Necrosis associated with K deficiency is usually dark in colour, and the necrotic areas become dry and brittle.

The yellowing and necrosis may be preceded by a light green interveinal mottle affecting mature to older leaves, and often most obvious on the leaves of axillary shoots. This may be the earliest, or only sign of potassium deficiency, but depending on cultivar and conditions, it may not develop. Initially, leaves of intermediate age may be more affected than the older leaves. Minor veins retain their green colour, dividing the pale tissue into small areas. In some cases, the interveinal tissue may become necrotic, either remaining as isolated spots or coalescing to form small, irregular lesions.

Potassium deficient crops tend to produce small, thin storage roots of poor quality. Orange-fleshed varieties usually have a paler-than-normal flesh colour.

Possible confusion with other symptoms

The appearance of an interveinal chlorotic mottle or small necrotic pits in interveinal tissue of leaves of intermediate age may resemble symptoms of manganese deficiency. However, in the case of manganese deficiency, the green zones around veins are broader and more diffuse. The absence of symptoms on the youngest leaves, and the occurrence of necrotic lesions on the oldest leaves indicate potassium deficiency.

On older leaves, interveinal patches of necrosis surrounded by yellow areas are also symptomatic of magnesium deficiency. Magnesium-deficient crops are generally pale all over, and the pattern of interveinal chlorosis is usually more regular, with the major veins remaining green for their entire length.

Diagnostic soil and plant tissue tests

The determination of a critical concentration for potassium in sweetpotato leaves is complicated by the fact that sweetpotato has some capacity to substitute sodium for part of its potassium requirement. In the presence of sufficient sodium, a critical concentration of 2.6% K has been determined in the 7th to 9th youngest leaf blades. In solution culture experiments with very low sodium concentrations, the critical concentration for K was found to be approximately 4.0%. The maximum substitution effect seems to be achieved at relatively low sodium concentrations, which would be found in most soils. Therefore, a critical concentration of 2.6% would apply in most field situations. This appears to be consistent with field measurements from crops which responded to potassium fertiliser.

Soil exchangeable potassium measurements are frequently used to predict the potassium status of crops, but they do not reflect the soilís reserves of potentially available potassium, which may be released over a period of time. Crop responses to potassium fertilisation are generally expected at exchangeable potassium values in the range 0.2 - 0.6 cmol(+)/kg soil, although this relationship depends on the soil texture and total cation exchange capacity (CEC): sandy soils may respond only in the range 0.05 - 0.25 cmol(+)/kg. In heavier soils, a positive correlation between sweetpotato yield and exchangeable potassium has been reported over a range of 0.2 - 1.0 cmol(+)/kg soil.

Management

Cultural control

Since sweetpotato crops remove a considerable amount of potassium from the soil (approximately 8 kg potassium in storage roots plus vines per 1 tonne of roots), continuous cropping without fertilisation can lead to the exhaustion of soil potassium reserves. Potassium deficiency occurs most commonly on sandy soils, which have a low capacity to bind cations such as potassium, and on highly weathered soils with low cation status.

Potassium can be added to the soil either in the form of inorganic fertiliser or in organic mulches and composts. Recommendations for fertiliser additions range from 80 to 200 kg K/ha. Organic mulches are much more bulky than inorganic fertilisers, therefore requiring more labour, but the efficiency of nutrient use by the crop may be higher. Mulches of fresh plant material have a higher potassium content than animal manures.

Two applications of potassium are often used, one before planting and one after four weeks of growth, with the fertiliser banded along the row. This practice is most advisable on light-textured soils which may lose a lot of potassium by leaching. It has been suggested that ploughing in potassium fertiliser to a depth of 40 cm may increase its efficiency, noting that it is the deeper roots which most actively provide potassium for storage root growth in the late stages of crop development. Such deep placement is rarely practised, other than by the burial of compost under mounds in some traditional production systems.

Excessive application of potassium may lead to magnesium or calcium deficiency, due to depression of the uptake of these elements. On sandy soils in particular, magnesium and calcium applications may be necessary in addition to potassium to maintain a favourable balance.

>References

Bourke, R.M. 1985b. Influence of nitrogen and potassium fertiliser on growth of sweet potato (Ipomoea batatas) in Papua New Guinea. Field Crops Research, 12, 363-375.

de Geus, J.G. 1967. Fertilizer Guide for Tropical and Subtropical Farming. Centre d'Etude de l'Azote, Zurich.

D'Souza, E. and Bourke, R.M. 1986a. Intensification of subsistence agriculture on the Nembi Plateau, Papua New Guinea 1. General introduction and inorganic fertilizer trials. Papua New Guinea Journal of Agriculture, Forestry and Fisheries, 34, 19-28.

D'Souza, E. and Bourke, R.M. 1986b. Intensification of subsistence agriculture on the Nembi Plateau, Papua New Guinea 2. Organic fertilizer trials. Papua New Guinea Journal of Agriculture, Forestry and Fisheries, 34, 29-39.

Edmond, J.B. and Sefick, H.J. 1938. A description of certain nutrient deficiency symptoms of the Porto Rico sweetpotato. Proceedings of the American Society for Horticultural Science 36, 544-549.

Floyd, C.N., Lefroy, R.D.B. and DíSouza, E.J. 1988. Soil fertility and sweet potato production on volcanic ash soils in the highlands of Papua New Guinea. Field Crops Research 19, 1-25.

Fujise, K. and Tsuno, Y. 1967. Effect of potassium on the dry matter production of sweet potato. In: Proceedings of the International Symposium on Tropical Root Crops, Trinidad, Vol. I, pp II- 20-33.

Goodbody, S. and Humphreys, G.S. 1986. Soil chemical status and the prediction of sweet potato yields. Tropical Agriculture (Trinidad) 63, 209-211.

Hill, W.A. 1989. Sweet Potato. In: Plucknett, D.L. and Spence, H.B. (eds.) "Detecting Mineral Nutrient Deficiencies in Tropical and Temperate Crops." Westview Tropical Agriculture Series, No. 7, pp 179-188, Westview Press, USA.

Ivahupa, S. 1997. Sodium substitution of potassium in tropical root crops. MAgrSc thesis, The University of Queensland, Australia.

Landon, R.J. 1991) (ed.) Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics. Booker Tate Ltd; Longman, London.

OíSullivan, J.N., Asher, C.J. and Blamey, F.P.C. 1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

OíSullivan, J.N., Asher, C.J., Halavatau, S., Blamey, F.P.C. and Yapa, L.G.G. 1997a. Nutrient disorders of sweet potato and taro: advances in diagnosis and correction in the Pacific. In: Akoroda, M.O. (ed.) Proceedings of the Sixth Triennial Symposium of the International Society for Tropical Root Crops - Africa Branch, Lilongwe Malawi, October 1995. International Institute for Tropical Agriculture.

Spear, S.N., Edwards, D.G. and Asher, C.J. 1978. Response of cassava, sunflower and maize to potassium concentration in solution. III. Interactions between potassium, calcium and magnesium. Field Crops Research 1, 347-361.

 

Contributed by: Jane O'Sullivan

Characteristics and occurrence

Symptoms

Confusion with other symptoms

Diagnostic tests

Management

References

 


Symptoms of K deficiency induced in a plant in solution culture, showing fine interveinal mottle on mid leaves, and yellowing and necrosis in an interveinal to irregular pattern on oldest leaves (J. O'Sullivan).

 

Normal shoot tip of a purple-tipped variety (right) and a K-deficient plant of the same variety, showing reduction in purple pigmentation (J. O'Sullivan).

 

Examples of interveinal chlorosis on mature leaves, with minor veins retaining a green margin (J. O'Sullivan).

 


Interveinal yellowing  of older leaves in a K-deficient crop (C. Asher).

Small brown necrotic spots following interveinal yellowing in a mature leaf (J. O'Sullivan).

Yellowing and brown necrosis spreading from margins between veins of older leaves (J. O'Sullivan).

Thin, under-sized storage roots from a K-deficient crop in PNG (R.M. Bourke).